data manipulation after file (excel) input in shiny

I have an Excel dataset that I want to manipulate after uploading it. Let's say in this case I would like to add a newcolumn <- 3, how can I get around with this? many thanks in advance.

ui.r

    library(shiny)


ui <- fluidPage(titlePanel("daily data manipulation"),
                sidebarLayout(sidebarPanel(
                  fileInput('file1', 'Choose xlsx file',
                            accept = c(".xlsx"))
                ),
                
                
                
                mainPanel(tableOutput(outputId = "mytable") 
                  
                  )))

server.r

server <- function(input, output) {
  
  

  data <- reactive({
    inFile <- input$file1
    if(!is.null(inFile)) {
      readxl::read_excel(inFile$datapath)   
    }
  })
  

 dat <- data()
 

 
 output$mytable <- renderTable({
   dat$newcolumn <- 3
   
 })
 
  }

Reactive expressions can only be consumed in a reactive context, so dat <- data() should be moved inside renderTable.

server <- function(input, output, session) { 
  data <- reactive({
    inFile <- input$file1
    if(!is.null(inFile)) {
      readxl::read_excel(inFile$datapath)
    }
  })

  output$mytable <- renderTable({
    dat <- data()
    if(!is.null(dat))
    {
      dat$newcolumn <- 3
      dat
    }
  }) 
}

Also, checking if the reactive value is null before adding the new column ensures that until the user uploads an excel file the new column will not be rendered.